[Pdns-users] remote backend

[Brian Candler]

On 29/11/2023 14:04, Alexis Fidalgo wrote:
> So, by now, i dont know what is making for a query to be answered and 
> another not (timeout) and in a retry is answered ok. (this is why i 
> thought on speed and considered the unix socket but now i know it’s 
> not that)

Put logging in your remote backend and show what queries it receives and 
how long it takes to respond to each one. Use these logs to check that 
the queries generated by PowerDNS are what you expect (it may make 
multiple requests for a single received DNS query).

You can also take PowerDNS entirely out of the problem by making a set 
of suitable test HTTP calls directly to your backend, for the same set 
queries that PowerDNS would generate. If you can prove that your backend 
is taking too long to answer them (on the first attempt at least), then 
you know where to investigate.

For example, it might be that MongoDB is doing a lot of slow disk seeks 
(is it spinning rust or SSD?) but once it has the answer, everything it 
needs is cached in RAM so it's much quicker on the second attempt. Or 
maybe it's not indexed properly. You really need to drill down further 
to prove or disprove that idea.

If you find that MongoDB is the bottleneck and can't be tuned, then 
there are other options. For example, if this database doesn't change 
very often, then you could write it out to a CDB file:

https://cr.yp.to/cdb.html
https://en.wikipedia.org/wiki/Cdb_(software)

This is optimised for very fast lookup with minimum seeking, and can be 
indexed in a single pass - but it can't be modified, so you'd have to 
regenerate the whole file periodically.  Also it has a 4GB size limit 
which is probably an issue here (limiting you to avg 14 bytes per 
key/value pair) so you may need to split into multiple files.

A suitably-indexed Postgres table with 300 million entries is big but 
not impossible, and PowerDNS could query it directly.